The present invention relates to a sealing between the nozzle and the mould of a continuous casting apparatus featuring at least one travelling flexible belt.
A familiar continuous casting apparatus of this type is characterized by a so called casting wheel which features a rim that is internally cooled by a liquid (U.S. Pat. No. 3,429,363). Along its circumference, the rim features a cavity corresponding to the dimensions of the desired casting and so forming three sides of the mould. The fourth side is formed by a metal belt touching the edges to the cavity along part of the wheel's circumference thereby creating a closed mould around the outside of the cross-section to be cast.
The belt is usually endless and runs on guide rolls which permit adjustment of the required tension. Thus, for a predetermined wheel diameter, the belt and casting wheel collectively form a mould of the desired length. Along the full length of the mould the outward side of the belt is intensively cooled by a liquid. When the casting wheel, by means of a drive, is caused to rotate, the belt moves along with it, thereby creating a mould that moves along with the cast material.
Another so called twin belt continuous casting apparatus is characterized by a pair of moving belts forming the mould in between.
Usually rotating and endless belts are used which run over appropriate guide rolls also used to stretch the belts. If a casting process allows for interruptions, it is also possible, instead of belts, to use strips of appropriate length running off a coil, being recoiled after passing the casting zone or serving as a covering layer for the cast material as it is moved on for further processing (DE Pat. No. 1 508 876).
It is known to build the side dams of the mould as travelling endless chains consisting of individual blocks which are tightly connected to each other by means of flexible joints. These blocks consist of metal or ceramic material and fit precisely into the space between the two belts, the width of the mould being determined by the space between the side dams located on both sides between the belts.
The path of the moving side dams can be positioned in a plane parallel or perpendicular to the axes of the guide rolls supporting the belts. The belts along with the side dams are usually activated by means of a drive connected to one of the guide rolls of the belts, whereby a mould moving along with the cast material is realized. It is also known practice to use an additional drive for the side dams.
The heat transferring from the cast material to the belt is removed through intense cooling of the belt's back side by means of a liquid. For a casting apparatus of the first type as well as for one of the second type, feeding systems are applied which direct the liquid metal into the mould. As a result of the heat drain in the mould, a completely or partially solidified casting--depending on the material being cast--will exit from the mould. So called open or closed feeding systems are in use. In an open system the liquid metal reaches the mould after flowing through an appropriate channel, the flow being controlled by familiar means. If the cast material has to meet high quality requirements, only a closed system can be used. In this case the liquid metal is fed into the mould by means of a nozzle which reaches into the mould at the same time sealing it off towards the entry side.
The material of the nozzle is chosen according to the properties of the liquid metal being cast. The requirements to be met concern temperature, heat-shock upon the first contact between nozzle and liquid metal, heat conductivity, erosion, chemical reactions with the liquid metal, formability and economy. By nature of the demands ceramic materials of various kinds, according to the specific requirements, are predominant. Nozzles being used consist e.g. of compressed and sintered ceramic-fibres based on silicon dioxide and alumina, impregnated with binder and filling material or of aluminum titanate, graphite, boron nitride, quartz etc.
Due to changes in dimension of the nozzle and the mould as a result of dilatation and heat-distortion, there is usually a certain clearance between these elements in order to avoid any jamming of the nozzle which could damage this vital part and cause serious problems for the casting process. This clearance usually amounts to approximately 0.1 to 0.5 mm. Due to this intended clearance, the metallostatic pressure in the liquid metal at the exit of the nozzle must be regulated with tight limits. Sealing gains rising significance with increasing pressure and/or decreasing viscosity or surface tension of the liquid metal. It is known practice to enhance the sealing by giving the nozzle an appropriate shape. Despite the measures described the danger of a backflow and its consequences remains.